This invention relates to rotary drill bits used to drill well bores in the earth for oil and gas production, and more particularly to such bits having at least one steel member and a plurality of cutting elements or inserts of hard metal secured in bores in the steel member in an interference press fit.
This invention involves an improvement over roller cutter drill bits of the "insert" type, such as shown for example in U.S. Pat. Nos. 2,687,875 and 4,105,263. These bits comprise a bit body having a threaded pin at its upper end adapted to be detachably secured to a drill string for rotating the drill bit and delivering drilling fluid under pressure to the drill bit, and a plurality of depending legs (typically three legs) at its lower end, each having a generally inwardly extending bearing journal. A roller cutter is rotatably mounted on each of the bearing journals. Each roller cutter comprises a generally conical roller cutter body having a recess in the base thereof for receiving the bearing journal and a plurality of cutting elements mounted on the conical surface of the body and engageable with the formation to be drilled. The roller cutter bodies are typically machined from a forged steel blank. The cutting elements are typically elongate members of sintered tungsten carbide and are secured to the roller cutter body by pressing the inserts in an interference fit into blind end bores or holes drilled in the roller cutter body. While drill bits of the above-described insert type have been generally satisfactory as evidenced by the fact that such bits have been a standard commerically available product in the drill bit manufacturing industry for over thirty years, it nonetheless suffers from several significant problems.
As best illustrated in FIG. 2 of U.S. Pat. No. 4,105,263, the blind end holes in the roller cutter body reduces the body's strength in compression thereby requiring that the roller cutter body be made thicker than otherwise required so as to withstand the downward loads (which may exceed 60,000 pounds) applied to the drill bit during drilling. Because the outer dimensions of a roller cutter body are limited by the size of the well bore to be drilled and the need for the roller cutter body to mesh with the other two cutter bodies of the bit, an increase in roller cutter thickness can be achieved only by a correspondence reduction in bearing journal size. The latter reduces the load carrying capability of the bearing journal which also must carry the downward load applied to the bit. Failure of one of the bearing assemblies (i.e., the journal, the roller cutter and any intermediate bearing means) of a drill bit, is one of the most common causes of bit failure and thus a limiting factor in the useful life of many bits. Moreover, bearing assembly failure can be more serious than other modes of bit failure, such as insert wear, in that it may result in a roller cutter body being left at the well bore bottom when the bit is pulled from the well bore for replacement. This roller cutter body must be removed by a time-consuming and hence costly "fishing" operation, before drilling can be resumed.
Another problem, which arises during manufacture of the roller cutter, is providing the proper interference fit of the inserts in the blind end holes in the cutter body. If the fit is too loose, the inserts will not be retained during drilling. Conversely, if the fit is too tight, high forces are generated in the roller cutter body when the inserts are pressed into the roller cutter body, and portions of the roller cutter body between adjacent inserts may fracture and be broken from the body during drilling. Accordingly, the dimensional tolerance of the blind end bores and inserts must be held within close limits.
Moreover, securing the inserts to the roller cutter body by an interference press fit, limits the number of materials of construction which may be used for the roller cutter body to those which will provide adequate core strength for the roller cutter body. In addition, it limits the closeness of the spacing between adjacent inserts and the diameter and hence the protrusion length of the tungsten carbide inserts, which have relatively low strength in bending.
As disclosed in U.S. Pat. Nos. 3,599,737 and 4,168,923, attempts have been made to provide additional means for retaining the inserts in the blind end holes to supplement the holding power of the interference press fit, thereby enabling certain of the above-described problems to be overcome to some extent. However, the additional retaining means have not proven entirely satisfactory and have not found wide commercial acceptance. In particular, the welding technique disclosed in U.S. Pat. No. 4,168,923 proved difficult to perform in practice, because the difference in thermal expansion between the tungsten carbide inserts and the steel roller cutter bodies and the brittle phases produced in the weld resulted in cracking of many of the welds upon cooling and because the disparity in elastic moduli across the weld resulted in cracking of other welds during use of the bit.
As shown in U.S. Pat. No. 4,381,825, the securing of cutting elements to a drill bit body by means of an interference press fit is also utilized in certain drag type drill bits. Such drill bits comprise a lower bit body portion or cutter head of steel having a plurality of blind end bores therein, and cutting elements, such as those commercially available under the trade designation "STRATAPAX" from the General Electric Company of Worthington, Ohio, pressed into the bores. Each cutting element comprises a layer of synthetic diamond deposited on a disc of tungsten carbide to form a so-called compact, and an elongate member or stud of tungsten carbide to which the compact is brazed. Drill bits of this type suffer from many of the same problems as those described above for the insert type roller cutter drill bits.